Topographic Contour Element Features in Patterned Macrostructures of the 18^th Century Persian Bulat Steel

A study of a late 18^th century Bebut bulat (Damascus) steel blade from Persian Kurdistan revealed its structure to consist of carbide banding in a troostite matrix. Reaching a width of 50 μm, the carbide bands were composed of accumulations of oblong particles of excess cementite. The bands of the continuous troostite matrix have a width of up to 100 μm. Banded (laminated) bulat structures are scored from 4 to 6 in terms of carbide heterogeneity. The internal microstructural composition of the Bebut bulat blade includes 58% straight sections of laminated banding, 22% wavy curved lines, 15% open reticular structures, and 5% closed reticular structures. The maximum phosphorus content of 0.4% is observed in troostite areas of carbide layers. In the troostite layers, the amount of phosphorus gradually decreases from 0.2% at the interlayer boundary to zero in the middle part of the troostite layer. The microhardness distribution plot of the bulat steel was experimentally determined to have a clearly expressed maximum of 1020 HV in carbide layers and minimum of 390 HV in troostite layers. This hardness-banded texture was determined to have a huge impact on the generation of a naturally undulating surface during the formation of a patterned bulat structure. The main bulat patterns are formed by individual topographic contour elements: cavities, basins, undulations, saddles, crosspieces, and wavy dunes. The bulat pattern thus represents a set of randomly alternating individual elements of the bulat structure in different local areas of the blade surface, which appears to have been a deliberate effect. Regularities in the local sections and the repetition of the basic patterns are described.